The present invention relates to a multi-cavity injection molding apparatus for optical disk substrates such as compact disks having recorded information by pits or magneto-optical disks from which recorded information is read by utilizing optical characteristics, magneto-optical interaction, etc.
In general, an optical disk substrate normally formed of transparent resin has a central hole 1 as shown in FIG. 2 regardless of the case that the substrate has an uneven surface by pits or it has a smooth surface without pits. The central hole 1 is adapted to be mounted to a rotary driving portion in an optical disk drive. It is required that the substrate surface around the central hole 1 is uniform in optical characteristics such as an index of double refraction. Therefore, in molding the optical disk substrate, it is obliged to provide an injection opening for injecting resin into a cavity of a mold at a position corresponding to the center of the optical disk substrate where the central hole thereof is to be formed. That is, it is necessary that the mold for the optical disk substrate is provided with a sprue for injecting the resin into every cavity.
The optical disk substrate is molded normally one by one by using an injection molding apparatus. However, such an injection molding apparatus for molding the optical disk substrate one by one is disadvantageous in mass productivity.
To solve this problem, it has been attempted to manufacture or employ an injection molding apparatus capable of simultaneously molding a plurality of optical disk substrates.
Such a multi-cavity injection molding apparatus is generally constituted of a mold unit having a plurality of cavities for molding the optical disk substrates and an injection unit for supplying a molten resin to the mold unit. The mold unit is provided with a plurality of resin feeding passages respectively communicating with the plural cavities or respectively connected to a plurality of sprues corresponding to the plural cavities.
In this kind of injection molding apparatus, it is necessary to carry out resin purging in order to eliminate a possibility that the molten resin may be decomposed in several hours after stopping operation of the apparatus. However, in the above construction of the injection molding apparatus, the resin purging is difficult, rendering the maintenance of the apparatus troublesome.
As another attempt in such a multi-cavity injection molding apparatus, the injection unit for supplying the molten resin to the mold unit is provided with a plurality of resin supplying sections or screw mechanisms for respectively supplying the molten resin under pressure to the plural sprues in the mold unit. However, as the plural screw mechanisms are provided, special design and manufacture of the injection molding apparatus are required to cause an increase in cost.